In recent years, as the crude oil processed in China is heavier increasingly, the raw materials processed by catalytic cracking are heavier and poorer; in addition, many enterprises have renovated their catalytic cracking plants or increased the operating severity of their catalytic cracking plants in order to attain a purpose of improving gasoline quality or increasing the yield, resulting in poorer quality of products obtained by catalytic cracking, especially catalytic diesel oil.
New hydrocracking processes and techniques for producing naphtha components and low-sulfur clean diesel fuel with a high added value by means of hydro-conversion of diesel oil with high aromatic content have good application prospects, in order to improve the utilization of petroleum resources, improve the overall level of quality of gasoline and diesel fuels, attain an objective of optimized product bending and maximized product value, and meet the increasing demand for clean fuels in China. Domestic and foreign researchers have also made extensive researches. There are reports on conversion of catalytically cracked light cycle oils into blended extra-low-sulfur diesel and high-octane gasoline components with hydrocracking techniques in foreign countries. For example, on the NPRA Annual Seminar 1995, David A. Pappal et al. introduced a single-stage hydrocracking process developed by Mobil, Akzo Nobel/Nippon Ketjen, and M. W. Kellogg. On the NPRA Annual Seminar 2005, Vasant P. Thakkar et al. introduced the LCO Unicracking™ technique developed by UOP. It is reported that both techniques can be used to convert low-value catalytic cycle oils into blended high-octane gasoline component and high-quality diesel component.
A key point in the catalytic diesel oil hydro-conversion process and technique is to accomplish ring-opening and cracking of di-aromatic hydrocarbons and tri-aromatic hydrocarbons in the catalytic diesel fraction while keeping the mono-aromatic hydrocarbons in the gasoline fraction and reducing ring-opening reaction of the aromatic hydrocarbons in the gasoline fraction and the gas produced through further cracking as far as possible, and thereby improve the yield and octane number of the gasoline product.
In addition, as that technique is applied in industrial application, the catalytic diesel oil conversion technique exhibits some drawbacks in industrial application: Firstly, compared with other hydrocracking techniques and processes, the catalytic diesel oil hydro-conversion technique (FD2G) and process results severe deviation in product distribution and product quality from the design objectives in the initial stage of operation, i.e., the octane number of the gasoline product and the gasoline yield are obviously lower than the desired targets; as the production time extends, the product distribution and the quality of gasoline product are gradually improved, till they reach a good and relatively stable level; however, that process is very long (usually longer than 1 month).
Secondly, compared with conventional hydrocracking techniques, in the catalytic diesel oil hydro-conversion technique, reaction raw materials have poor quality and high contents of di-aromatic hydrocarbons and tri-aromatic hydrocarbons; moreover, the reaction conditions are demanding. Consequently, the catalyst deactivation rate in the catalytic diesel oil hydro-conversion process is much higher than that in conventional hydrocracking processes, resulting in a shortened operation cycle, and bringing difficulties to production scheduling in the plant.
CN105642335A has disclosed a method for preparing a hydrocracking catalyst, comprising: (1) selecting a hydrocracking catalyst support material, which contains at least one acidic cracking material, adding an acidic peptizing agent into the support material, and preparing a hydrocracking catalyst support through molding, drying, and roasting; (2) preparing a saline solution with 10 to 30 g/100 ml active metal content, impregnating the hydrocracking catalyst support prepared in the step (1) in the saline solution in a saturated state, and then drying and roasting the hydrocracking catalyst support; (3) impregnating the roasted catalyst obtained in the step (2) in a liquid olefin in a saturated state, and heating the impregnated catalyst at 50 to 400° C. for 1 to 70 h in air, so that a carbon deposition reaction happens on the catalyst and thereby a carbonized catalyst is obtained; (4) loading the carbonized catalyst support prepared in the step (3) directly into a muffle furnace preheated to 400 to 600° C., and roasting for 5 to 200 minutes, to burnt off the carbon deposit on the surface layer of the catalyst support, so that the carbon amount in the support accounts for 10 to 90% of the total carbon amount before the catalyst support is roasted; (5) preparing a saline solution with 40 to 80 g/100 ml active metal content, impregnating the roasted catalyst support obtained in the step (4) in the saline solution in a saturated state, and then drying and roasting, to obtain a finished product of hydrocracking catalyst. In that method, the active metal is impregnated in two steps, to form gradient distribution of active metal on the catalyst, wherein, the active metal content in the surface layer of the catalyst is higher than the active metal content in the core part of the catalyst; thus, the catalyst can be used to process wax oil raw material to produce chemical raw materials such as tail oil and heavy naphtha, etc., increase the hydrogenation saturation rate of the macromolecular tail oil and decrease the saturation rate of the generated naphtha fraction, and thereby improve the reaction selectivity. However, the catalyst is not applicable to catalytic diesel oil hydrocracking.
It can be seen that there are drawbacks in the prior art, including: poor reaction effect of fresh catalyst in the initial stage of operation, and poor catalyst stability.